I. Field of the Invention
The present invention relates generally to mass gas flow sensors and, more particularly, to a hot wire gas flow sensor.
II. Description of Related Art
There are many previously known mass gas flow sensors which provide analog output signals proportional to the mass of gas flow through the sensor. One such type of previously known mass gas flow sensor is known as a hot wire sensor. Such hot wire sensors are frequently used in the automotive industry.
In the previously known hot wire sensors, the hot wire sensor includes a housing having a throughbore through which the gas flows. Both a hot wire and a cold wire are positioned within a bypass bore in the housing while an analog electronic circuit maintains a temperature differential between the hot wire and cold wire at a predetermined amount. For gasoline engines in which the sensor measures the mass of the air/fuel mixture, the temperature differential between the hot and cold wire is typically maintained at 200xc2x0 C. by varying the current flow through the hot wire.
In practice, gas flow through the housing bore cools the hot wire. Consequently, in order to maintain the temperature differential between the hot and cold wire, the current flow through the hot wire is increased by the electronic circuit in an attempt to maintain the constant 200xc2x0 C. temperature differential between the hot and cold wire. The current flow through the hot wire in effect forms a signal proportional to the mass gas flow through the sensor housing bore.
While these previously known mass gas flow sensors have proven adequate in the automotive industry where the flow rate of the air/fuel gaseous mixture, or alternatively just the air intake flow rate, these previously known flow sensors have presented special problems for measuring other types of gas flows, such as the gas flow for hydrogen, propane, methane and other combustible fuels.
One disadvantage of these previously known flow sensors is that, by maintaining the temperature differential between the hot and cold wire at the conventional 200xc2x0 C. differential, is that potential combustion or ignition of the gas through the sensor is possible. This is particularly true where the sensor is used in environments where the ambient temperature of the gas is relatively high.
A still further disadvantage of these previously known flow sensors is that failure of any one of several different sensor components may result in excessive current flow through the hot wire. This excessive current, in turn, heats the hot wire to an elevated temperature possibly causing combustion of the gas flow through the sensor.
A still further disadvantage of these previously known hot wire flow sensors is that a relatively lengthy warm up time is required before the hot wire reaches its operating temperature. This in turn results in inefficient operation of whatever device, e.g. a fuel cell or engine, that is operatively coupled with the fuel sensor.
Additionally, it has been difficult, and therefore expensive, to seal these previously known flow sensors from gas leaks around the post which supports the leads leading to both the hot and cold wire sensor under high pressure situations.
The present invention provides improvements in gas flow sensors which overcomes all of the above-mentioned disadvantages of the previously known devices.
In brief, the gas flow sensor of the present invention comprises a housing having a fluid passage which is coupled in series with a passage through which the measurement of the gaseous flow is desired. The sensor housing typically includes a bypass passageway so that only a portion of the gas flow through the sensor housing passes through the bypass passageway.
In the conventional fashion, a post is secured to the housing and extends radially inwardly into the bypass bore so that an inner end of the post is positioned substantially centrally within the bypass bore. Both a hot wire constructed of an electrical resistive material as well as a cold wire are secured to the free end of the post while the electrical leads from both the cold wire and hot wire extend through the post and to control circuitry associated with the flow sensor.
Consequently, in the conventional fashion, a temperature differential between the hot wire and cold wire is maintained at a predetermined amount by the control circuitry by varying the current flow through the hot wire in an amount necessary to maintain this temperature differential. The current flow through the hot wire is then proportional to the mass gas flow through the sensor and this current flow through the hot wire provides an output signal representative of that mass gas flow rate.
Unlike the previously known flow sensors, however, the temperature differential between the hot wire and cold wire is maintained at a preset amount in the range of 30xc2x0 C. to 100xc2x0 C., and preferably substantially 65xc2x0 C. Due to the high thermal conductivity of many gases, such as hydrogen, propane, methane and the like, a relatively low temperature differential between the hot wire and cold wire in the range of 30xc2x0 C. to 100xc2x0 C. is sufficient to provide an accurate measurement of the gas flow rate through the sensor.
In certain situations resulting from component failure of either the control circuitry or failure of the cold wire, excessive current flow through the hot wire can result thus resulting in excessive heating of the hot wire and possible combustion of the gas flowing through the sensor. In order to eliminate the possibility of such combustion, the present invention provides several hardware as well as software techniques to prevent the flow of excessive current through the hot wire.
In one embodiment a Zener diode is connected in parallel across the positive end of the hot wire and ground so that the voltage imposed across the Zener diode is proportional to the voltage across the hot wire. Consequently, whenever the voltage drop across the hot wire, and thus the current flow through the hot wire, exceeds a predetermined amount, the Zener diode conducts and prevents the further increase of current through the hot wire.
In another form of the invention, a driving transistor is utilized to provide current flow from the power source to the hot wire. In the event of failure of the driving transistor, a Zener diode clamped either across the emitter or collector of the driving transistor, or optionally across the base, is utilized to limit current flow through the driving transistor and consequently current flow through the hot wire.
Still other means are disclosed for limiting the current flow through the hot wire in the event of failure of one or more components of the control circuitry for the sensor and/or the failure of the cold wire. For example, in one embodiment, a software control is utilized to sense the voltage drop across the hot wire and then limit any further increase of the current flow through the hot wire under software control.
The present invention further provides circuitry for augmenting the current flow through the hot wire following the electrical energization of the hot wire. Such augmentation is advantageous in that it provides rapid heat up of the hot wire so that the hot wire reaches its operating temperature more quickly. In one embodiment, an RC timing circuitry is alternatively connected between the power source and the hot wire or between the power source and the base of the driving transistor for the hot wire. This RC timing circuit increases the current flow to the hot wire as a function of both the capacitance and resistance of the RC timing circuit. Alternatively, however, augmentation of the initial startup current to the hot wire can be obtained through software control or by other means.
The present invention further provides enhanced sealing of the lead wires from both the hot wire and cold wire through the housing post. In the preferred embodiment of the invention, a cavity is formed within the post through which the lead wires for both the hot wire and cold wire extend. This cavity is then filled with a sealing material thereby preventing leakage of gas along the lead wires for the hot wire and cold wire.
Additionally, both the hot wire and cold wire are preferably sealed to protect the hot and cold wire from the gas flow through the sensor housing. Glass, polyamide, epoxy or other sealing means can be used to seal the hot and cold wires. Additionally, special materials, such as gold, stainless steel or the like may be used for the lead wires for both the hot wire and cold wire.